To be so cutting is not natural (5)
I believe the answer is:
sharp
'not natural' is the definition.
('sharp' is the opposite to 'natural')
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for sharp that I've seen before include "Acute; pointed" , "Cutting; acidic" , "Quick-witted - keen" , "Punctually; raised note" , "Cutting; note" .)